Interest in alternative fuels such as liquified petroleum gas ("LPG") has increased in recent years due to the inherent cost and environmental advantages over other fuels. LPG has particularly received much attention as an alternative to gasoline or diesel for use in internal combustion engines. Propane, the primary constituent of LPG, is a byproduct of the refining of gasoline, and it is a byproduct of the transfer of natural gases in pipelines. It is readily available and at costs far below that of gasoline.
LPG was recently listed under the Clean Air Act in the United States as a suggested alternative fuel because it is more environmentally compatible than gasoline. LPG burns more completely, producing less carbon monoxide and hydrocarbon emissions. Also, using LPG as a fuel reduces the emission of volatile organic compounds which occurs during gasoline refueling.
The United States Federal Government recently promulgated legislation, referred to as Corporate Average Fuel Efficiency ("CAFE") standards, to promote the use of more environmentally compatible fuels. CAFE created a system of incentives which encourages manufacturers to build automobiles and trucks which use alternative fuels such as LPG. As a result, there is increased interest in manufacturing and retrofitting automobiles and trucks with LPG fuel systems.
The injection of liquid fuels such as gasoline into internal combustion engines is well known (see U.S. Pat. No. 4,700,891). Such fuel injectors create fine atomization of liquid fuel, which improves the efficiency of the burning cycle.
Although LPG in its gaseous form has been used as a reasonably effective fuel in internal combustion engines, there is an associated reduction in power capability as compared to liquid LPG fuels. This power reduction is mainly due to the reduced amount of air and fuel which can be drawn into the intake manifold when the LPG enters the manifold in gaseous form.
With liquid LPG, a further gain in peak power (and simultaneous reduction in the emission of nitrous oxides) results from the cooling of air and fuel within the intake manifold from vaporization of injected LPG. This also reduces the tendency for engine knock.
Use of LPG in liquid form as a fuel is fairly new in the art. However, several obstacles are associated with attempting to inject liquid LPG directly into the intake manifold of an internal combustion engine. In particular, it is difficult to maintain LPG in its liquid state near the heated engine compartment. LPG has a very low boiling point (see FIG. 4 for the liquid-vapor phase boundaries for propane and isobutane, the primary constituents of LPG). Even under pressure, LPG will tend to bubble or boil as the boiling temperature at a given pressure is approached. The formation of bubbles, often called "champagning" or "flashing," can cause inconsistent injection and poor air/fuel ratio control.
Various approaches to cooling LPG to ensure injection of a homogeneously liquid fuel have been attempted. Examples include using metered LPG to cool the injected fuel to a liquid state (see U.S. Pat. No. 4,489,700), and employing a spool tip at the injection nozzle to cool the injected LPG (see U.S. Pat. No. 5,076,244). However, previous approaches have in large part been unsuccessful, particularly in maintaining the LPG injected into the manifold in a fully liquid state throughout the operating range of the engine, as for example at idle.
Consequently, it is clear that an improved, efficient and effective LPG injection system which can maintain the injected LPG in a fully liquid state throughout the operating range of an internal combustion engine continues to be needed.